1. Field of the Invention
This invention relates generally to electronic circuits having a plurality of input ports and, more particularly, to a circuit that is responsive to a selected one of a plurality of input signals.
2. Description of the Prior Art
In a typical electronic process control system, a plurality of sensors provide electrical signals that are each representative of a specific physical phenomena. One sensor may, for example, provide a voltage that is proportional to temperature. Another sensor may provide a voltage that is proportional to humidity, etc.
In one type of electronic process control system, the electrical sensor signals are applied to a central processor via a multiplexer. The multiplexer is a plurality of switches that are operable to cause a selected one of the sensor signals to be provided to the processor. Accordingly, in this type of system, a sensor signal path from a sensor to the processor includes the multiplexer.
The multiplexer is often comprised of either relays or field effect transistor (FET) switches. A relay is typically large, costly and short lived, in comparison to an FET switch. The FET switch, however has an "on" state that inserts an undesirably high resistance into the sensor signal path. Additionally, the FET switch has an "off" state that inserts a leakage current into the sensor signal path. The undesirable resistance and the leakage current both vary with changes in ambient temperature. Moreover, not all FET switches can be used when the sensor signal is comprised of voltages in a range of +10 to -10 volts, a usual input voltage range in electronic process control systems.
In another type of electronic process control system, all of the sensor signals are provided to preamplifiers. Amplified sensor signals from the preamplifiers are applied to the processor via the multiplexer. Accordingly, an amplified sensor signal has a signal path from the output of a preamplifier to the processor via the multiplexer. Typically, this type of system is not suitable for processing low level signals because the preamplifier generates undesired error signals. The preamplifier generates undesired error or offset signals. The error and offset signal vary as a function of temperature. The error and offset signals may be reduced for a given preamplifier by a temperature compensation circuit. Duplicating typical offset or error correction circuits becomes undesirably complex where many channels are used. The preamplifiers also draw input bias current when not selected. This causes undesirable interaction when the inputs of the preamplifiers are connected together to share inputs or feedback. Circuitry to selectively remove power for an entire preamplifier can become undesirably complex.
Accordingly, there is a need for a new circuit that is operable over a wide range of ambient temperature to provide one of a plurality of input signals to a load without generating the error signals. Additionally, it is desired that the new circuit be responsive to a range of input signals commonly encountered in electronic process control systems. Also the new circuit should have inputs that draw no input current and exhibit very high input impedance when not selected.